In hybrid vehicles and electric vehicles, a vehicle drive battery is configured so as to output a prescribed drive voltage, and it is necessary to always monitor its output voltage. For example, a vehicle drive battery of a hybrid vehicle may produce an output voltage of about 200 V, and it may be boosted to about 500 V for use. In connection with this, a voltage monitoring circuit for watching for occurrence of an abnormal voltage is necessary. In recent years, a high-voltage monitoring circuit for watching for occurrence of an abnormal voltage that is higher than 1,000 V is demanded.
For example, JP-2009-201192-A and JP-2012-095427-A are related to an example motor drive device. FIG. 8 shows an example motor drive device 100. A high-voltage battery B is mounted on a vehicle body while being insulated therefrom. In the motor drive device 100, a boost converter 101 boosts a DC high voltage (e.g., 200 V) that is output from the battery B (to 600 V, for example), and an inverter circuit 103 converts the boost voltage into a 3-phase AC voltage for motor drive via a smoothing capacitor 102. The 3-phase AC voltage is supplied to a vehicle drive motor M.
To monitor the boost voltage, the motor drive device 100 is equipped with a voltage detection circuit 200. The voltage detection circuit 200 detects voltages at nodes b1 and b2 which are connected to the positive terminal and the negative terminal of the battery B, respectively. And, a control circuit (not shown) controls motor driving by outputting control signals the boost converter 101 and the inverter circuit 103 on the basis of detection results of the voltage detection circuit 200. As shown in FIG. 9, the voltage detection circuit 200 may be formed by an operational amplifier 201 and resistors 202a-202e. 
As shown in FIG. 9, the series-connected resistors 202a and 202b divide a positive-side high voltage of the battery B. One terminal B1 is connected to the node b1 which is connected to the positive terminal of the battery B (see FIG. 8), and the other end is grounded to the vehicle body. The series connection point of the resistors 202a and 202b is connected to the non-inverting input terminal of the operational amplifier 201.
On the other hand, the series connected resistors 202c and 202d divide a negative-side high voltage of the battery B. One terminal B2 is connected to the node b2 which is connected to the negative terminal of the battery B (see FIG. 8), and the other end is grounded to the vehicle body. The series connection point of the resistors 202c and 202d is connected to the inverting input terminal of the operational amplifier 201.
The resistor (feedback resistor) 202e is provided to set an amplification gain of the operational amplifier 201. One end of the resistor 202e is connected to the inverting input terminal of the operational amplifier 201, and the other end is connected to an output terminal OUT of the operational amplifier 201. A detection signal that is output from the voltage detection circuit 200 is input to the control circuit (not shown), and the control circuit outputs control signals for controlling operations of the boost converter 101 and the inverter circuit 103, whereby driving of the motor M is controlled.
To manufacture a voltage detection circuit for detecting a high voltage to be used in a motor drive device of a hybrid vehicle or an electric vehicle, an integrated circuit chip consisting of an operational amplifier and resistors may be formed by a regular semiconductor device manufacturing process, and the integrated circuit chip may be mounted on a lead frame and sealed with resin. In this case, the resulting voltage detection circuit may not function if discharge occurs between high voltage application leads or between these leads and other, neighboring leads.
In view of this, it may be considered to employ a high-withstand-voltage structure as shown in FIG. 10 with a wide mounting area. Specifically, an operational amplifier integrated circuit 302 and plural chip resistors 303 are mounted on a mounting board 301, and are connected to each other by interconnections (not shown).
If a voltage that is higher than 600 to 1,000 V is applied to the voltage detection circuit, assuming that the resistance of each chip resistor 303 is 620 kΩ the number of resistors 303 to be mounted becomes as large as about 30 to 80. As each chip resistor 303 has a structure that a metal coating resistance element is formed on a ceramic substrate and measures about 2 mm×1 mm, the size of the mounting board may become a several centimeter square or a little larger than a 10 centimeter square. Thus, it is difficult to implement this structure in small size.